Full sized scattering bar alt-PSM technique for IC manufacturing in sub-resolution era

ABSTRACT

A process is described for shrinking gate lengths and poly interconnects simultaneously during the fabrication of an integrated circuit. A positive tone photoresist is coated on a substrate and is first exposed with an alternating phase shift mask that has full size scattering bars which enable a gate dimension to be printed that is ¼ to ½ the size of the exposing wavelength. The substrate is then exposed using a tritone attenuated phase shift mask with a chrome blocking area to protect the shrunken gates and attenuated areas with scattering bars for shrinking the interconnect lines. Scattering bars are not printed in the photoresist pattern. The process affords higher DOF, lower OPE, and less sensitivity to lens aberrations than conventional lithography methods. A data processing flow is provided which leads to a modified GDS layout for each of the two masks. A system for producing phase shifting layout data is also included.

This is a division of patent application Ser. No. 10/205,790, filingdate Jul. 26, 2002 now U.S. Pat. No. 6,711,732, Full Sized ScatteringBar ALT-PSM Technique For IC Manufacturing In Sub-Resolution Era,assigned to the same assignee as the present invention.

FIELD OF THE INVENTION

The invention relates to a method of fabricating an integrated circuitin a microelectronic device. More particularly, the present inventionrelates to a method of forming smaller lithography patterns onsubstrates by using phase shifting masks.

BACKGROUND OF THE INVENTION

The fabrication of integrated circuits and semiconductor devicesrequires the application of a lithography process to define a pattern ona substrate. A photosensitive layer on a substrate is patternwiseexposed with radiation that passes through a mask having opaque andtransparent regions. A commonly used mask called a binary mask iscomprised of an opaque material such as chrome or chromium on atransparent substrate which is typically quartz. The electric field oraerial image that exits from the mask during an exposure has a highintensity corresponding to light passing through transparent regions anda low intensity where light has been blocked by opaque regions. Theaerial image is projected onto a photosensitive film called aphotoresist to form exposed regions and unexposed regions. A smallamount of radiation does reach “unexposed” regions, especially nearborders with exposed regions because of diffracted light. This conditionlimits the minimum size of the features that can be formed or resolvedin the photoresist pattern. Since there is a constant demand fordecreasing feature size in order to build faster circuits or for ahigher density of circuits per unit area, numerous resolutionenhancement techniques have been developed during the past severalyears.

The minimum feature size that can be printed for a given process isdefined as R=kλ/NA where R is the minimum resolution, k is a constantfor the process, λ is the exposing wavelength, and NA is the numericalaperture of the projection optics in the exposure tool. A combination oflower k and lower λ coupled with a higher NA has enabled a steadyreduction in technology nodes in recent years from over 250 nm to 180nm, 130 nm, and now to 100 nm. Traditionally, k is reduced byenhancements to the mask or lithographic process such as attenuatedmasks, off-axis illumination (OAI), optical proximity correction (OPC),scattering bars (SB) and high contrast photoresists.

The most popular exposure tools have been g-line (436 nm) and i-line(365 nm) steppers and scanners but Deep UV (248 nm) tools have beenimplemented to achieve resolution in the 130 nm to 250 nm range.Currently, 193 nm exposure tools are being accepted as the primary pathto the 100 nm node. With each step in λ reduction, the NA has beenmaximized to be in a range of about 0.7 to 0.85. If all the advances ink, λ, and NA are combined, the minimum feature size that can be achievedis about half wavelength. In other words, for the current technologybased on 193 nm exposure tools, the smallest feature that can bereliably produced in manufacturing is about 90 to 100 nm. There is aneed to push the imaging capability toward quarter wavelength with newoptical enhancements since the time gap between each technology node isbecoming shorter and exceeds the ability of tool manufacturers to matchthe pace with λ and NA improvements. This is especially true for gateelectrodes in transistors where the feature size is already sub-100 nmand is rapidly decreasing.

Attenuated phase shift masks have been widely introduced intomanufacturing processes because they can enable a smaller resolutionfeature to be printed with a larger process window than with binarymasks. One example is found in U.S. Pat. No. 6,210,841 in which anattenuated mask is formed by adding an attenuator material such asMoSiO_(X)N_(Y) to portions of the substrate. The thickness of theMoSiO_(X)N_(Y) is adjusted so that the phase of the light is shifted by180° in regions where radiation passes through the attenuator.Scattering bars are also used to improve the resolution of the processto 130 nm.

A photoresist process is also characterized by its process latitude.That is the combination of focus and exposure dose settings that willgenerate a photoresist feature within a given linewidth or space widthtolerance which is usually within ±10% of a targeted value. A focuslatitude or depth of focus (DOF) of at least 0.4 microns and preferablynear 1 micron or larger is desirable for an acceptable manufacturingprocess. At the same time, the dose latitude or acceptable range ofexposure doses should be at least 10% (±5% about a target dose) andpreferably 15% or greater. As an example, a gate feature size of 180 nmthat is printed at a dose of 20 mJ/cm² and with a process having a DOFof 1 um and a dose latitude of 20% means that a feature sizes between162 nm and 198 nm can be printed if the exposure dose stays within arange of 18 mJ/cm² to 22 mJ/cm² and the focal plane does not shift frombest focus by more than 0.5 micron (μm) in either direction (towardplane of substrate or away from substrate). A phase shifted mask canhelp to increase process latitude besides improving resolution. This isvaluable because expensive rework involving stripping the resist layer,recoating and re-exposing when linewidth is out of specification can bereduced which leads to a lower cost device.

An alternating phase shift (alt-PSM) approach first proposed by Levensonin 1988 appears to be the best method of achieving a quarter wavelengthresolution. In the case of an alt-PSM with a single trench etched intothe substrate as illustrated in FIG. 1 a, exposing radiation 2 passesthrough two transparent regions with thicknesses t₁, and t₁+t₂ onopposite sides of an opaque region 4. The amount of quartz 6 that hasbeen removed in the second transparent region to form trench 7 isthickness t₂ that has been determined according to an equation(n−1)×(t₂)=½λ where n is the refractive index of the quartz 6 and λ isthe exposing wavelength. The result is that the phase of light exitingregion with thickness t₁ (through aperture B) is 180° out of phase withthe light exiting the adjacent region with thickness t₁+t₂ (throughaperture A). The phase width is shown as the distance PW. Thisrelationship forms a higher contrast aerial image that exposes thephotosensitive film on the substrate and allows smaller feature sizes tobe printed with an alternating mask 3 than with a binary mask which hasa uniform thickness of quartz and light of only one phase exitingapertures of the mask.

An alt-PSM 3 with a dual trench is shown in FIG. 1 b. Trench 8 with adepth t₃ and trench 9 with a depth t₄ are etched into quartz 6.Distances t₃ and t₄ are adjusted so that radiation 2 that passes throughalt-PSM 3 exits aperture C with a phase θ° and exits aperture D with aphase (180+θ)°. The distance (t₂, t₃, or t₄) that a trench is etchedinto a mask substrate will hereafter be referred to as a phase depth.

However, fabrication of alt-PSMs has been difficult in terms ofautomatic phase assignment, phase inspection and repair, and cycle timewhich increases cost beyond what many IC manufacturers can afford.Alt-PSM has a phase conflict problem because it creates dark resist(unexposed positive tone photoresist) at all areas corresponding to a 0°to 180° transition in the mask. Meanwhile, automatic phase assignmentand alt-PSM design rule for an IC layout are very crucial for realapplications. Spence (U.S. Pat. No. 5,573,890) and Wang (U.S. Pat. Nos.5,858,580 and 6,228,539) reveal double exposure alt-PSM to overcomethese concerns. Spence uses an alt-PSM and a structure mask in order toachieve a new gate length and remove unwanted dark lines formed by thephase shift method. However, this solution does not avoid the difficultyof making the mask. It also does not address polysilicon interconnectlines that must shrink in dimension simultaneously with a smaller gatesize in order to realize the full benefit of smaller gates. Wang uses adark field phase shifting mask for shrinking gate length and a structuremask to define polysilicon interconnects as well as protecting theshrunken gate. There is no alt-PSM rule checking to resolve phaseconflicts and the structure mask is binary with a relaxed design rulethat prohibits features near quarter wavelength from being printed.

Another alt-PSM technique is described in U.S. Pat. No. 5,994,002 and isprimarily concerned with mask corrections to control an opticalproximity effect where lines of equal size on the mask are separated bydifferent space widths. Uncorrected masks tend to print isolated lineslarger than dense lines in resist films. While the method deals withoptical proximity effects, it does not mention correcting for lensaberrations or adjusting two different features on a mask which areprinted simultaneously such as gate lines and polysilicon interconnects.

Still another alt-PSM method described in U.S. Pat. No. 5,882,827combines alt-PSM and attenuated PSM features on the same mask toovercome alignment concerns. The mask appears difficult to producebecause in some cases, the attenuated shifter material is overlaid onalt-PSM regions where the transparent quartz has been removed to adjustits thickness relative to the non-shifted transparent regions. Thisarrangement would be difficult to inspect and repair and thereforecostly to make.

For the 100 nm technology node that is currently being implemented inmanufacturing, gate lengths as small as 60 or 70 nm are being producedand there is no method with current lithography techniques to achievethis dimension. Typically, 100 nm gate features are printed in resistand then trimmed by a plasma etch to achieve a smaller size. This methodrequires an extra step of etching which can be costly and difficult tocontrol if two or more feature sizes are trimmed at the same time.Therefore, it is desirable to have a lithography technique that is ableto print features in photoresist that are about one quarter the size ofthe exposing wavelength. The method should provide an acceptable processwindow and be compatible with a manufacturing process flow. A logicallayout management flow should be provided so that an appropriate mask ormasks can be built.

SUMMARY OF INVENTION

One objective of the present invention is to provide a method forforming a feature in a photoresist layer that has a smaller dimensionthan can be produced by conventional resolution enhancement techniques.This is especially true for gate lengths.

A further objective of the present invention is to provide a method ofshrinking two different features simultaneously such as a gate lengthand a polysilicon interconnect in an integrated circuit.

A still further objective of the present invention is to provide a dataprocessing flow for defining a layer in an integrated circuit. The dataenables an improved alt-PSM mask to be built in addition to a trim maskwhich together can be used to print shrunken gate and interconnectfeature sizes.

A still further objective of the present invention is to provide asystem for producing phase shifting layout data which can handle alt-PSMdesign rules, optical proximity corrections (OPC), scattering bar rulesand gate blocking area generation for tritone attenuated PSM maskmaking.

These objectives are achieved through the design of an alt-PSM with fullsize scattering bars (FSSB) and an attenuated tritone mask hereafterreferred to as att-PSM. The masks are employed in a double exposuremethod in which the alt-PSM with FSSB is used to primarily define ashrunken gate structure in a first exposure. The scattering bars enablea smaller phase width as illustrated in FIG. 6 b that effectively permita smaller gate size to be controllably manufactured. Phase height andphase width can be adjusted to improve depth of focus, especially forrelaxed pitches and isolated lines. The optical proximity effect (OPE)is reduced by introducing the FSSB and this allows a higher flexibilityin mask design. In addition, the effect of lens aberrations such asx-coma is minimized with the new alt-PSM which results in higherfidelity photoresist features, especially those with dimensions belowthe half wavelength size.

The chrome section of the tritone att-PSM protects the gate featuredefined with the alt-PSM by preventing the erasure of phase shiftingregions and preventing the creation of undesirable artifact regions thatwould otherwise be formed by the alt-PSM exposure. The scattering barfeature which is combined with the attenuated portion in the tritoneatt-PSM trims the polysilicon interconnect line to a smaller dimensionthan can be achieved with a binary mask. This enables the polysiliconinterconnect to be shrunk at the same time as the gate feature. Theblocking area (chrome region) of the att-PSM is based on the phaseassignment of the alt-PSM. Examples of the alt-PSM with FSSB and thetritone att-PSM are found in FIGS. 3 a and 3 b, respectively.

A comprehensive data management flow is crucial to building qualitymasks for this double exposure approach. The system for producing phaseshifting data is a command script comprised of alt-PSM design rules,OPC, scattering bar rules, and gate blocking area generation for thetritone att-PSM. The management flow involves first reading in thelayout of diffusion (underlying active area) and poly layers. Next,design of the alt-PSM mask follows a sequence of alt-PSM rule check,assigning FSSB for small linewidths, assigning phases along gate area,and inputting OPC data. The next step is inputting the blocking area forshrunken gates including the assigned phase areas and polygate areas.Then the trim mask is designed by inserting data for polysiliconinterconnects, the block area, and full size scattering bars that isgenerated through a set of parameters such as scattering bar size andseparation between main pattern and scattering bar. After OPC data isdetermined and corrections are inputted, the final output is a modifiedGDS layout for the each mask. The GDS layout is subsequently used by themask fabricators to produce the masks.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a and 1 b are cross-sectional views of conventional alternatingphase shift masks.

FIGS. 2 a–2 b show a layout extraction method for mask making accordingto an embodiment of the present invention.

FIGS. 3 a–3 b are top-down views of phase and trim masks used in thedouble exposure method of the present invention.

FIGS. 4 a–4 b are top-down views of photoresist patterns on a substrateafter the masks from FIGS. 3 a–3 b were used in a single exposurelithography process.

FIG. 5 is the resulting pattern in a photoresist layer after a doubleexposure method using the masks depicted in FIGS. 3 a–3 b.

FIG. 6 a is a top-down view of a conventional alternating phase shiftmask.

FIG. 6 b is a top-down view of an alternating phase shift mask with fullsize scattering bars (FSSB) according to the present invention.

FIG. 7 shows an x-coma aberration in an aerial image from a conventionalalt-PSM that is corrected by an alt-PSM with FSSB of the presentinvention.

FIG. 8 is a flow diagram showing the data management sequence during adesign of an alt-PSM with FSSB and a trim (tritone att-PSM) mask of thepresent invention.

FIG. 9 is a graph that illustrates how phase height influences depth offocus at various pitch sizes in a conventional alt-PSM.

FIG. 10 is a graph that shows how an alt-PSM with FSSB can be optimizedto improve depth of focus over a conventional mask for certain pitchsizes.

FIG. 11 is a graph showing the optical proximity effect (OPE) for aconventional alt-PSM at different phase heights.

FIG. 12 is a graph showing how OPE is reduced by using an alt-PSM withFSSB.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a method by which smaller feature sizes inphotoresist layers on substrates can be printed than are currentlyavailable from other optical lithographic methods. An alternating PSMapproach is modified and combined with an optimized attenuated PSM withcapability for printing feature sizes at one quarter of the exposingwavelength. The invention is also a method by which the aforesaid masksare designed and a system for producing the phase shifting layout data.

In one embodiment, a method for layout management flow 50 is provided asillustrated in FIG. 8. Initially, data representing the location of theunderlying diffusion (active area) which is indicated by the dashed line12 in FIG. 2 a is loaded into layout processing software. This softwarecan be commercially available EDA IC design software or a set of scriptprograms. The layout for polysilicon line 14 in FIG. 2 a is also loadedinto the software program according to step 52 in the flow sequence. Thealt-PSM is then generated in step 54 by first checking the alt-PSMdesign rules for phase conflict between 0° and 180° phase regions orbetween θ° and (180+θ)° phase regions. These rules include a set ofparameters for perpendicular gate, inter-digital gate, poly to polypitch and line-end cap. Full size scattering bars (FSSB) shown asfeatures 16 in FIG. 2 b are assigned with linewidths W₁ that can beequal to or greater than the width W₀ of the gate feature 10. Thenphases along the gate area are assigned with phase width W₂ and a phasethat is either θ° or (180+θ)° where θ is from 0 to 180. All phase widthsW₂ are equivalent. Other parameters such as phase extension, minimumphase width, phase spacing, and overlay offset are also included.

To enlarge the depth of focus (DOF) for printing a photoresist patternwith this alt-PSM, the linewidth W₁ of the FSSB 16 is resized by anamount that can vary in increments of about 10 nm according to patternpitches. This adjustment may be a minor modification of FSSB size but isable to increase DOF by a significant amount.

Next optical proximity correction data is generated and inputted tomodify the layout. OPC data can be rule-based or from an empiricalresist model to assure that the fidelity of the feature printed inphotoresist is close to the original IC layout.

Data for the gate block area is then processed in step 56. This blocklayer includes the assigned phase areas and polygate areas (true gate)to prevent erasing of the shrunken gate features. The gate block alsoapplies to the att-PSM structure fabrication which maintains a chromefilm on top of shrunken gate areas from the alt-PSM exposure to preventlight leakage from attacking these gates. The remainder of the trim(att-PSM) mask is designed in step 58. Features such as polysiliconinterconnect lines 14, the block area (not shown), and full sizescattering bars 18 in FIG. 2 b are generated through a set of parameterssuch as scattering bar 18 size and separation between the main pattern14 and the scattering bar 18. The attenuated part of the tritone masksuch as scattering bar 18 is comprised of a semi-transparent half-tonefilm on a substrate. The half tone film can be formed from a materialsuch as MoSiO_(X)N_(Y). The substrate is quartz when the exposingradiation is Deep UV (248 nm), i-line (365 nm) or g-line (436 nm) and ispreferably CaF₂ when the radiation has 193 nm wavelength. Thetransparency of the attenuated stack is from about 1% to about 30% andthe phase change is 180° compared to the phase exiting the substratethrough regions with no attenuated layers.

The scattering bars 18 on the att-PSM can be either sub-resolution orfull size scattering bars and have a width L₂ that is between 33% and100% of the dimension L for line 14 (FIG. 2 a). When scattering bar 18is sub-resolution, it is not printed in the photoresist pattern. Fullsize scattering bars do print artifacts in a photoresist pattern.However, these artifacts are not printed in a double exposure methodsince the region containing the artifacts is exposed during the alt-PSMexposure. The scattering bar (SB) is used to improve depth of focus andresolution of the trim mask compared to one without SB. The SB isassigned according to a set of parameters that define its size,separation from the main feature, and how many scattering bars are alongthe main feature. The SB 18 is usually a distance of 1L to 2L away frominterconnect 14 and can consist of one to three bars. The layout for thetrim mask then goes through OPC generation and corrections are appliedto form a modified GDS layout for the trim mask. Likewise, a modifiedGDS layout is done for the alt-PSM.

The final step 60 in the layout management flow is to send the GDSlayouts to the mask fabricators. In this case, an alt-PSM 21 with FSSBis produced as shown in FIG. 3 a and an att-PSM 23 with SB is producedas shown in FIG. 3 b. Alt-PSM 21 may be a single trench or dual trenchtype as shown in FIGS. 1 a and 1 b, respectively. When alt-PSM 21 is asingle trench type, the mask has opaque chrome regions 20, transparentsubstrate regions 22 that transmit light that exits the mask with 0°phase, and transparent substrate regions 24 having a thinner substratethickness and transmitting light that exits the mask with 180° phase. Itshould be noted that phase shifted regions 24 are formed during the maskmaking process by a selective etch process and it is important that thephase depth t₂ as shown in FIG. 1 a, is uniform over the entire mask.Otherwise, the phase of light exiting through aperture B will either beslightly larger (180°+θ) or slightly smaller (180°−θ) than the intended180°. As a result of phase depth variations, the features printed in aphotoresist are likely to be shifted in size and location from theirintended placement.

When alt-PSM 21 is a dual trench type, shifts of θ° where θ is from 0 to180 are designed into the layout and transparent regions 22 transmitlight that exits the mask with a θ° phase while transparent regions 24transmit light with a (180+θ)° phase. A high quality alt-PSM is onewherein the fabrication process allows the phase depths t₂, t₃, t₄ inFIGS. 1 a and 1 b to be controlled such that the phase shift ismaintained within±3° of the intended shift in degrees across the entiremask. Phase height H₁ is defined as the height of the phase aperture oropening in regions 22 and 24 in FIG. 3 a.

Att-PSM 23 in FIG. 3 b is tritone because it consists of three distincttypes of regions which are the chrome blocking area 27 having a width W₃that is greater than W₀ in FIG. 2 b, clear transparent regions 25, andattenuated regions 26 and 28 for the interconnect line and scatteringbar, respectively. Interconnect 26 and scattering bar 28 are bothcomprised of an attenuator material such as MoSiO_(x)N_(y) which hasbeen deposited on the mask substrate. A less desirable option is toconstruct a binary mask where interconnect 26 and scattering bar 28 arechrome but a binary mask does not have the resolution or process windowthat is afforded by an att-PSM.

A second embodiment of the present invention is a system for producingphase shifting layout data and consists of a command script comprisingalt-PSM design rules, OPC, SB rules, and gate blocking area generationfor tritone att-PSM. Initially, a single line transistor gate in anintegrated circuit is identified. As shown in FIG. 2 a, the gate 10 isgiven a width W₀ and length L_(G) wherein the length L_(G) (longer sideof the rectangular feature) is orthogonal to the two longer sides of anunderlying rectangle 12 that represents the active area. The lengthL_(G) of the gate 10 in the phase mask design is greater than the widthW_(A) of the active region 12 by an extension amount that is sufficientto compensate for possible misalignment between masks carrying the phaseand trim layouts. Then phase shifting areas are assigned for defining aphase mask having a single line transistor gate and first opaque areas.

For alt-PSM, a layout including phase shifting areas in an opaque fieldand another layout of phase 0° and phase 180° or phase θ and phase(180+θ)° are provided. The adjacent phase shift region must always beadjusted so that adjacent phase shift regions are 180° out of phase witheach other. Those skilled in the art will recognize how to adjust themask making process to provide phase regions of θ° and (180+θ)° for adual trench type alt-PSM.

Then a gate block layer is defined which involves assigning phase areasand polygate areas (true gate) to prevent erasure of shrunken gates.This gate block information is also useful data for att-PSM maskfabrication that maintains a chrome film on top of shrunken gates toprevent light leakage from attacking the gates. Next a trim mask likethe one shown in FIG. 3 b is defined that includes interconnect 26 (polylayer excluding gate areas), block area 27 (a slight demagnification ofthe phase area), and sub-resolution or full size scattering bar 28 thatare generated through a set of parameters such as scattering bar sizeand separation between the main pattern and the scattering bar. Thewidth L of interconnect 14 in the design stage is larger than width L₂of the scattering bar and larger than W₀ of the gate.

When scattering bar 28 is sub-resolution on the mask, it is not printedin resist but improves the DOF and resolution of the lithographyprocess. If a full size scattering bar 28 is incorporated on the mask,then DOF will be increased more than with a sub-resolution scatteringbar 28 but artifacts will be printed on the substrate. However, the FSSBartifact location is exposed by the phase mask and therefore will notremain in the photoresist pattern in the case of a double exposuretechnique.

The opaque gate block area 27 includes phase shifted areas on the phaselayout and polygate on active area. Its size is demagnified somewhat forphase layouts and true gates in order to compensate for possiblemisalignment between first and second masks. Finally, these layouts gothrough OPC generation and then a modified GDS layout is provided formask fabrication.

In a third embodiment, the present invention is a method of formingshrunken gates simultaneously with shrunken polysilicon interconnects inan IC manufacturing process. The method involves producing an alt-PSMwith FSSB and an att-PSM with SB using a data management flow and systemfor producing phase shifting layout data as described in the first twoembodiments. An alt-PSM 21 that can be either a single or dual trenchtype is provided as shown in FIG. 3 a with chrome regions 20, 20 a, and20 b, transparent regions 22 that transmit light which exits the maskwith phase θ°, and transparent regions 24 that transmit light whichexits the mask with phase (180+θ)°. Phase shifted regions 24 alternatewith regions 22 and are spaced apart by chrome regions 20 a and 20 b.

Chrome region 20 a is a shape that will be used to form a gate structureon a substrate. Chrome regions 20 b correspond to full size scatteringbars that are not printed in a photoresist layer but help to shrink thegate size and improve its process latitude compared to a conventionalalt-PSM. The pattern in FIG. 3 a is from a small section of the mask 21and is repeated across alt-PSM 21.

A further comparison of a conventional alt-PSM and an alt-PSM with FSSBis provided in FIG. 6 a and FIG. 6 b. Conventional alt-PSM 40 as shownin FIG. 6 a has chrome lines of width D₁ corresponding to poly gatesthat separate alternating transparent regions 42 and 44 representing θ°and (180+θ)° phase areas, respectively, wherein θ is from 0 to 180. Thephase width PW is the distance between adjacent chrome lines 43 and thepitch is the distance (PW+D₁) between one edge of one chrome line andthe same edge of an adjacent chrome line 43. In an alt-PSM 46 with fullsize scattering bars 45 illustrated in FIG. 6 b, the chrome linecorresponding to the gate feature has width D₁ as before. However, fullsize scattering bars 45 having a width D₂ and comprised of chrome arepositioned between each chrome line 43. D₂ is typically equal to orgreater than D₁.

Between each pair of chrome lines 43, there is a θ° phase area 42 and a(180+θ)° phase area 44 that have equivalent phase widths PW₂ and PW₁,respectively. A finer resolution of the resulting gate in a patternedphotoresist film can be achieved with mask 46 because the phase widthsPW₁ and PW₂ can be made smaller than PW in mask 40.

A higher contrast aerial image is produced by mask 46 which enables ahigher resolution and larger process window to be achieved in thelithographic patterning of substrates. An additional one or two fullsize scattering bars can be inserted between adjacent chrome lines 43 inphase mask 46 but the number is limited by the physical space. Forinstance, smaller pitches in the printed photoresist pattern in a rangeof about 250 nm to about 600 nm may only allow one FSSB 45 betweenadjacent chrome lines 43 in the alt-PSM. For larger pitches, a totalranging from one to three FSSB 45 may be placed between each chrome line43. It should be noted that the size of the chrome lines 43 are largeron the mask than the resulting lines which are printed in thephotoresist. Typically, the exposure system provides a 4× or 5×reduction in pattern size so a chrome feature on the mask will appear inthe printed photoresist pattern as a feature ¼ or ⅕ of the original masksize.

A tritone att-PSM 23 (FIG. 3 b) is also provided which has a chromeblocking area 27, highly transparent substrate regions 25, andattenuated regions 26 and 28 that allow a transmission between 1% and30% of the exposing wavelength. Scattering bar 28 can be eithersub-resolution or full size and has a width L₂ that is about 33% to 100%of the size of the width L of interconnect 26. When scattering bar 28 issub-resolution in size, it is not printed in a photoresist layer butdoes help to print interconnect 26 at a smaller size and with a largerprocess window than is possible with a binary mask. In the case of afull size scattering bar 28, a larger process window is also realizedand the artifacts that are printed by the trim mask in a single exposuremode are removed by the exposure with the phase mask in a doubleexposure method.

The alt-PSM 21 and the att-PSM 23 are used in a double exposure processof a substrate 30 coated with a positive tone photoresist film 32. In apositive tone photoresist, exposed regions become soluble in an aqueousbase developer and are washed away while unexposed regions are insolublein developer and remain on the substrate. Therefore, transparent regionson a mask become regions clear of photoresist on a developed substrate.FIG. 4 a shows the resulting photoresist pattern on substrate 30 if onlyalt-PSM 21 is used during an exposure.

Alt-PSM 21 generally has transparent regions comprised of quartz and theexposing radiation consists of one or more wavelengths selected from arange of about 150 nm to 600 nm and is typically VUV (157 nm), 193 nm,Deep UV (248 nm), i-line (365 nm) or g-line (436 nm) radiation. Alt-PSM21 typically has transparent regions comprised of CaF₂ when the exposingwavelength is 157 nm or 193 nm. As a result of the lithographic processinvolving patterning of photoresist 32 on substrate 30 with alt-PSM 21,clear substrate regions 34 are formed where exposed photoresist waswashed away by developer. The four rectangular features 34 at the top ofFIG. 4 a are equivalent in size and shape even though they were formedby exposure with light of two different phases that are θ° and (180+θ)°.Regions 32 are unexposed photoresist.

FIG. 4 b depicts a pattern in photoresist 32 on substrate 30 that wouldresult from a single exposure with att-PSM 23. Clear regions 34 areformed corresponding to transparent regions in att-PSM 23 whilephotoresist 32 remains in areas corresponding to chrome or attenuatedregions on att-PSM 23.

A key point in the present invention is that photoresist 32 on substrate30 is not exposed with a single exposure but with two exposures followedby a single development step. First, photoresist 32 on substrate 30 isexposed with radiation through alt-PSM 21 using radiation selected froma wavelength or wavelengths in the range of about 150 nm to about 600 nmand then photoresist 32 is exposed with att-PSM 23 using a wavelength orwavelengths selected from a similar range. The wavelength used for thefirst exposure does not have to be the same wavelength used in thesecond exposure. However, throughput may be increased if the twoexposures are performed in the same exposure tool at the samewavelengths. Substrate 30 is developed in aqueous base to give a patternillustrated in FIG. 5. Because of the overlap of the two patterns fromthe two exposure steps, photoresist 32 only remains as a shrunken gatefeature 32 b and as a shrunken interconnect 32 a. All other areas 34 onsubstrate 30 are clear of photoresist since they were exposed at leastonce.

Gate 32 b has a width W₄ that is smaller than can be printed with binaryor attenuated PSM masks in combination with resolution enhancementtechniques. Moreover, interconnect 32 a has a width L3 that has beenshrunk by employing an att-PSM 23 with a scattering bar 28. Thus, bothgate 32 b and interconnect 32 a have been shrunk simultaneously whichhas not been achieved in prior art. At this point, the photoresistpattern is typically plasma etch transferred through the underlyingsubstrate which is normally polysilicon and then the photoresist isremoved by a conventional stripping method.

Other benefits of an alt-PSM with full size scattering bars over aconventional alt-PSM are summarized in FIG. 7 and FIGS. 9–12. FIG. 7shows that a conventional alt-PSM as represented by FIG. 6 a is muchmore sensitive to lens aberrations than an alt-PSM with full sizescattering bars as depicted in FIG. 6 b. In both cases, the chrome lines43 have a D₁, that will form 80 nm wide lines in a photoresist patternwith a pitch of 600 nm, the NA of the exposure tool is 0.60 and the 193nm exposing wavelength has a coherence (σ) of 0.3. For the modifiedalt-PSM as shown in FIG. 6 b, a scattering bar 45 has been inserted witha width D₂ that is 1.75 times larger than D₁.

In this example, the lens has an x-coma aberration and the response of aconventional alt-PSM is a merging of two adjacent phase regions in theaerial image exiting the mask as illustrated by curve 48 in FIG. 7 whichwill result in a degraded photoresist pattern. The contrast that wasgained by having alternate phase shift regions has been lost because ofthe x-coma. On the other hand, the alt-PSM with FSSB of the presentinvention is not affected and the high contrast aerial image is retainedas shown by the four distinct peaks in curve 49 corresponding to fouradjacent phase regions 42, 44 in FIG. 6 b, two with phase θ° and twowith phase (180+θ)°. Since all exposure tools have some lensaberrations, an alt-PSM with full size scattering bars will be anadvantage over conventional alt-PSM because it will be able to achieve asmaller resolution feature in exposed photoresist because of a lowersensitivity to aberrations like x-coma, y-coma and other higher orderaberrations.

Referring to FIG. 9, the phase height H₂ in FIG. 6 a in a conventionalalt-PSM is adjusted and the resulting DOF is shown for phase heights H₂between 370 nm and 1500 nm. Note that the resulting photoresist patternshave some “forbidden pitches” with sizes between about 250 nm and 380 nm(0.25–0.38 μm) where the DOF cannot be optimized much above 0.5 micronsby this alt-PSM.

In FIG. 10, DOF can be increased relative to a conventional alt-PSMusing an alt-PSM with FSSB especially for relaxed pitches where thespace width between photoresist lines is at least 5 or 6 times largerthan the linewidth. In this example, the targeted photoresist line sizeis 80 nm and for pitches of 500 nm size or greater, the alt-PSM withfull size scattering bars provides a higher DOF (curve 110) than theconventional alt-PSM (curve 100).

Another benefit afforded by an alt-PSM with FSSB is a smaller opticalproximity effect. As shown in FIG. 11, the linewidth or criticaldimension (CD) of a line printed in photoresist is plotted on the y-axisand the pitch containing the line is plotted on the x-axis. Thelinewidths of the intended photoresist lines are 80 nm for all pitches.The exposing wavelength is 193 nm from a 0.6 NA tool with σ=0.3. Theresults are plotted for various phase heights ranging from 370 nm to1500 nm. For all phase heights, the line is printed smaller than thetargeted 80 nm (0.08 μm) at all pitches below 0.35 μm size and reaches amaximum width of about 100 nm at a pitch size of 0.40 μm. Ideally, thetarget CD should be printed as an 80 nm line at all pitches. To adjustfor the actual results printed in photoresist, a separate correction isusually made for each pitch on the mask so that by exposing a correctedmask, the CD printed in photoresist will be about 80 nm for all pitches.The maximum optical proximity correction is expressed as the differencebetween the largest and smallest CDs printed from the uncorrected maskas illustrated in FIG. 11. Here the correction for all the curvescorresponding to different phase heights ranges from about 55 to about60 nm. A large correction is undesirable for mask designers since itrequires more space on the layout that could otherwise be used to packcircuits more closely together.

In FIG. 12 the conventional alt-PSM is replaced by an alt-PSM with fullsize scattering bars and the optical proximity effect data is generatedusing the same exposure conditions as described for FIG. 11. As with theconventional alt-PSM, the printed linewidth is small for a 200 nm pitchand increases as pitch increases to 350 nm. However, in this example themaximum line size printed is about 90 nm at a 350 nm pitch. As the pitchchanges from 350 nm to 375 nm, the printed line size decreases and thenincreases again as the pitch grows from 375 nm to 550 nm. The correctionfor all the curves with different phase heights ranges from about 45 toabout 50 nm which is less than the values for a conventional alt-PSM.This improvement allows a greater flexibility in mask design whichultimately pays off in more circuits per substrate and a lowermanufacturing cost.

Therefore, the third embodiment of the present invention enhancesresolution and DOF for both gate and interconnect features which is animprovement over prior art that improves resolution only for gatefeatures. In addition, the quality of the image that is printed inphotoresist is improved over conventional alt-PSM because of a betterimage control that results from a lower sensitivity to lens aberrations.The incorporation of full size scattering bars leads to lower opticalproximity corrections and a greater flexibility in mask design.

While this invention has been particularly shown and described withreference to, the preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made without departing from the spirit and scope of this invention.

1. A method of forming a feature on a substrate comprising: performing afirst exposure, with an alternating phase shift mask with full sizescattering bars, on the substrate; and performing a second exposure,with a tritone attenuated mask having at least one wherein said fullsize scattering bars are not printed in the feature on the substratefrom the second exposure.
 2. The method of claim 1 wherein said firstexposure is comprised of patternwise exposing a positive tonephotoresist layer on said substrate through said alternating phase shiftmask with scattering bars using radiation comprised of one or morewavelengths from about 150 nm to 600 nm.
 3. The method of claim 1further comprised of a single development step which follows the secondexposure.
 4. The method of claim 2 wherein the feature is comprised ofline widths that are between ¼ and ½ of the exposing wavelength in size.5. The method of claim 1 wherein said alternating phase shift mask iscomprised of phase shift regions having a phase width that are separatedby chrome regions.
 6. The method of claim 5 wherein said chrome regionsare comprised of chrome lines that are used to define shrunken gates inthe feature and full size scattering bars wherein from one to threescattering bars are positioned between each chrome line.
 7. The methodof claim 6 wherein each chrome line and full size scattering barseparates a θ° phase shift region from a (180+θ)° phase shift regionwherein 0 is from 0 to
 180. 8. The method of claim 6 wherein said fullsize scattering bars have a width that is equal to or greater than thewidth of said chrome lines that define the shrunken gates.
 9. The methodof claim 6 wherein said θ° phase shift region and said (180+θ)° phaseshift region have phase widths that are equivalent.
 10. The method ofclaim 1 wherein the tritone attenuated mask is comprised of atransparent region, an attenuated phase shift region, and a chrome blockregion that protects a shrunken gate formed by said first exposure. 11.The method of claim 10 wherein said attenuated phase shift region iscomprised of an interconnect line feature having a width and ascattering bar which is a line parallel to said interconnect linefeature and separated therefrom by a certain distance.
 12. The method ofclaim 11 wherein said scattering bar has a width that is 33% to 100% ofthe width of said interconnect line feature.
 13. The method of claim 11wherein said certain distance between said scattering bar and saidinterconnect line feature is from about one to two times the width ofsaid interconnect line feature.
 14. The method of claim 11 wherein saidscattering bar is sub-resolution in width.